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Understanding UNOS and exceptions from it
I'm studying for my EASA PPL(A) exam, and one thing that I came across are the following questions:

Q: An aircraft in the northern hemisphere intends to turn on the shortest way from a heading of 270° to a heading of 360°. At approximately which indication of the magnetic compass should the turn be terminated?
A: 330°

This one is clear as, by UNOS rule (Undershoot North, Overshoot South), I should undershoot in this case by 30°.
The next question sounds like this...

Q: An aircraft in the northern hemisphere intends to turn on the shortest way from a heading of 030° to a heading of 180°. At approximately which indicated magnetic heading should the turn be terminated?
A: 210°

This one is also clear: applying the UNOS-rule, I will need to overshoot by 30°.
But the next one is causing me understanding issues:

Q: An aircraft in the northern hemisphere intends to turn on the shortest way from a heading of 360° to a heading of 270°. At approximately which indication of the magnetic compass should the turn be terminated?
A: 270°

Here I see that the UNOS rule is not applied anymore, but I don't really get why.
By researching I found the following explanation in my Pooley's Flying Training [1] book:

The above allowances ["Undershoot on North. Overshoot on South" - op. ed] should be reduced:

when turning with bank angles less than 30°; and
when turning onto headings well removed from north and south - in fact, when turning onto due east or due west, no allowances need to be made.

I'm thinking that for that last question this second exception would apply, but I am not really understanding why.

Because the compass works correctly on east/ west magnetic headings (or on the magnetic equator).1
Suggestion: learn more about how the "dip" of the earth's magnetic field, combined with the fact that aircraft normally bank to turn, is the fundamental cause of the tendency for the compass to "lag" while turning if the aircraft's current heading contains a northerly component, and to "lead" while turning if the aircraft's current heading contains a southerly component, in the northern (magnetic) hemisphere.
The key factor is not whether the aircraft is turning toward the south or the north, but rather whether the aircraft's current heading contains a northerly or southerly component, and how much.  For example, when flying SSW, the compass will "lead" regardless of whether the aircraft is turning left toward a heading of S, or turning right toward a heading of SW. Conversely, when flying NNE the compass will "lag" regardless of whether the aircraft is turning left toward a heading of N, or turning right toward a heading of NE. So as an aircraft's current heading approaches due (magnetic) W or E, the compass error drops toward zero, regardless of whether the aircraft is turning from a more northerly heading (i.e. turning toward the south), or is turning from a more southerly heading (i.e. turning toward the north).2
Understanding the key role played by the aircraft's current instantaneous heading, rather than the direction of turn, is probably the real key to clearing up the OP's confusion.  It seems the OP believes the "UNOS" rule is based on the direction of turn, which is not the case--the key factor is the aircraft's current instantaneous heading.
But ultimately a pilot is usually going to be modulating the turn rate by the artificial horizon (via the bank angle) and/or the turn "coordinator" or the  turn rate indicator, in which case the behavior of the compass is not of great interest until the aircraft is getting close to the target heading, when the compass will be used as a guide as to when to roll out of the turn.  Hence the focus of the test questions on how the compass will be behaving as the aircraft approaches the target heading.
New suggested knowledge test question for OP-- if you are turning to the right (toward the north) from magnetic 180 to magnetic 225, according to the correct application of the "UNOS" rule, should you "undershoot" by rolling out "early" (e.g. do you expect the compass to "lag"?)  Or should you "overshoot" by rolling out "late" (e.g. do you expect the compass to "lead"?)  If you recognize that the key factor is the target heading, not the direction of turn, you'll know the answer.  225 is heading with a southerly magnetic component, so you should expect the compass to "overshoot" as you approach this heading, and so you should continue the turn until the compass has gone slightly past the target heading.
Secret knowledge: there are some specific cases where the compass's tendency to "lead" while turning when the heading contains a southerly component (in the northern magnetic hemisphere) can actually be used to a pilot's advantage.  It tends to give an "early warning" of the first beginnings of a bank, which can be helpful.  For more, see this related ASE answer.
Footnotes:

Within limits.  There is also the rotational inertia of the compass card itself to consider.  If you are in a steep turn, with the compass behaving erratically and often "freezing" or even reversing its direction of spin, you certainly would not expect the compass to read correctly the instant your actual heading passes through due east or due west.  That's why we keep the bank angle shallow when we are relying on the magnetic compass.  And that's one reason a heading with a large southerly component affords a much larger margin for error as per (the non-standard technique mentioned in) the last paragraph above.  The critical thing is that the aircraft's heading not be allowed to gain any significant northerly component, where the compass will "lag" rather than "lead", and the odds of that are best if the pilot chooses a target heading with a large southerly component.

Same as footnote 1.

Related ASE answer of interest, with some useful links -- Why do we undershoot/overshoot with a compass turn? -- but beware, in places the answer emphasizes the direction of the turn rather than whether the target heading is a northerly or southerly heading.  This may perpetuate the misunderstanding that has apparently ensnared the OP.

Of course we could think of all kinds of exotic variations-- for example, aircraft is flying 090 degrees, whips into a tight turn to right through 180 degrees and on through 270 and then rolls out on 275 degrees-- as it was was turning through the southerly headings, the compass might have surged far enough ahead that is still leading slightly, rather than lagging, by the time the a/c rolled out on a heading with a slight northerly component.  That's why we keep the bank angle shallow when flying with the compass-- and that's why the test questions tend to stick to fairly simple cases.


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Calculate Block Fuel based on Altitude, Temp, Taxi Fuel, Time for climb/approach and landing
I'm preparing for my EASA exams and one of the questions I came across for the "Flight Performance and Planning" exam sounds like this:
For planning a VFR flight, the following data are given:
Flight time with planning "overhead-overhead" = 2h 43min
Pressure Altitude = 6.500 ft
Temperature = ISA-20
Power setting = 2300 RPM
Taxi Fuel = 2 USG
Additional time for climb = 7 min
Additional time for approach and landing = 10 min
The reserve fuel has to be 30% of trip fuel.
Determine the minimum block fuel:
A. 47.3 USG
B. 50.4 USG
C. 43.8 USG
D. 39.2 USG
The correct answer is B. 50.4 USG.
The exercise also gives the following chart as a help:

My thinking was the following:

Based on the chart and for the given OAT, Altitude I can detect the Fuel Flow of 12,4 GPH
If I am subtracting 7 min climb time + 10 min approach and landing time, I get a Trip Time of 2h 26min
Based on The Trip Time and the Fuel Flow I can calculate that the Trip Fuel is ~28 USG
30% of the Trip Fuel would be about 8,4 USG, representing the Reserve Fuel
Adding Taxi Fuel + Trip Fuel + Reserve Fuel = 38 USG

Now, of course, something I am calculating wrong or terrible wrong, for sure, the missing part is the fuel needed for T/O and Landing... but I just don't know how to get that.

I think you are correct in climb and descent fuel being the missing pieces.  Climbing at full power for 7 minutes isn't too hard, just interpolate the fuel flow roughly between sea level and 6000'.  Descent is a little harder because most pilots don't descend at full power.  Nor do they cruise at full power...

@MichaelHall, even if I add the Fuel needed for T/O and Landing, the numbers don't add up to 55.4 USG. For T/O and Landing I calculated 17 minutes x 0.206 USG/minute = ~3.5 USG

I think the "tricky" part of this is the climb/approach time.  The "overhead to overhead" reference gave me a clue that maybe they meant you'd climb, then fly 2:43, then descend, so you would add rather than subtract the climb/landing times.

I think Max R (in a since-deleted answer, "2:43 + :7 + :10 = a convenient 3.0 hrs") is right about adding the times to get 3 hours. Then,

3 hours x 12.4 GPH = 37.2 gal
37.2 Gal x 1.3 = 48.36 gal
Add 2 gallons taxi fuel = 50.36 gal
Rounded gives 50.4 gal

Apparently they don't want you to multiply 1.3 x the taxi gas. (Although if you do, you get a higher number than any of the answers, which hints that you should try excluding those 2 gallons from the 1.3x multiplier and recalculating.)
I'll add that, from an actual flight planning perspective, using the fuel flow at cruise altitude times some number of minutes as the calculation for fuel used in climb & in descent seems a little non-sensical. As a way to make the math in the question easier, it works, but there's no obvious reason why fuel used in the climb is equal to 7 minutes of fuel at cruise. You can certainly pick some values where it just happens to work out like that, but that isn't how real flight planning works.
Especially if you want to claim 1/10th of a gallon precision in your answer!

Ah!  I did that (deducted the taxi fuel from the reserve) in one of my trial-and-errors, but then did the math wrong!  I would edit out your reference to my answer, and then I will delete mine.

@MaxR Edited to incorporate your formulation of the 3 hours total, thanks.

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What does mean CD, FD and MD on the Jeppesen LTBR VORB chart?

Hello all,
I just wanted to ask the meaning of CD068, FD068 and MD068. 068 is the course but i cannot find any info about CD,FD and MD.
Also, at the VOR A 25R chart, I saw FS250. Can you share their meaning? Thanks!

They are database identifiers for waypoints that aren't named.

MX### and MD### waypoints are missed approach waypoints
FD### waypoints are final approach fixes
CD### waypoints occur further out on the approach; I believe that the "C" stands for "course", as in a waypoint that defines part of the approach course.
FS### waypoint seems to be an FAF for, specifically, a VOR approach, and is designated when there are multiple approaches that would otherwise use duplicate identifiers.

More info on pages 8 & 9 at the link that DeepSpace posted.

According to Jeppesen's "NAME CONVENTIONS WAYPOINT IDENTIFIERS", in a VOR approach the CD, FD and MD points are FACF (Final Approach Course Fix), FAF (Final Approach Fix) and MAP (missed approach fix) respectively.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Why are there two Decision Altitudes at LTBR Bursa Yenişehir?

Hello,
As you can see, at the missed approach point, the chart gives 984 feet. On the other hand, The Decision altitude given as 959 feet. What is the reason ? Thanks.

Related: Although the missed approach point in the profile view is 1.1 DME-400 feet, the GS out reference altitude is given as 410 feet in 1.0 DME. HOW?

The glideslope altitude at the LOC MAP is 984. That’s not the DA.

Isn't the LOC MDA 1750? What's the point of including this extra figure?

@Ben The MDA is 1750, but if you were flying the ILS (and could execute the 5.4% climb out) then the altitude serves as a check just prior to hitting the DA (959). Likewise, if you were leveling at 1750 on the LOC it would provide a good indication for how much lower the normal 3 degree glidepath is to the runway at the MAP (should you get the runway in sight).


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What is the dots before Missed App Point on the Jeppesen VOR Chart?
What is the dots before Missed App Point on the Jeppesen VOR Chart ? and also why the pull up vector before the missed approach point ? Thanks.

This obviously isn't from the US with a CDFA approach.  European or Canadian?

This chart belongs to Bursa Yenisehir Airport - Turkey LTBR

The dots are a continuation of the glidepath to the runway.  This procedure is designed to follow the CDFA approach concept.  Constant Descent Final Approach.
Above the profile view is a table showing DME distances and altitudes.  You can see it in the attached image above.  The idea is to start the descent at the 9.0 DME inboard towards the VOR and to try and hit the altitudes on the table for each 1NM.  If you can do this, you will be on a constant descent to the runway which is a 3.30° glidepath.
The missed approach arrow is before the MAP because if you continue at MDA after that point, you may not be in a position to land within the touchdown zone on the runway.  This is part of the CDFA concept.
I can't tell from the attached picture above but the minimums may have (MDA) or  (DA) listed.  If it is a MDA (which it probably is) increase the altitude listed by at least 50' to make it a derived decision altitude (DDA).  This allows you to descend on a constant descent and if you don't have the airport/runway in sight at the DDA, to execute the missed approach without going below MDA.

This isn't a US chart.  However, usually on a U.S. chart, there will be a note stating "Only authorized operators may use VNAV DA(H) in lieu of MDA(H)."  That note is found in the profile view and should be seen in the attached picture but it isn't.  Until we can look at the exact plate asked about it is impossible to know for sure.

What is missing is the country this is for.  That will tell us a lot about the MDA or DA issue.  I am assuming since it is a CDFA approach, it will have DA minimums.  And since it isn't from the U.S. I doubt that note would be there.


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Why are there altitude differences on the Jeppesen ESB Ankara VOR chart?
What is the reason for the 10 feet difference between ESB dme altitude and profile view? Thanks.

Are you able to post the Turkish version of this chart?

Unfortunately, since that is not the current chart, the following is speculation.
The altitudes in the table are the recommended altitudes for a continuous descent. The altitudes in the profile view are step down altitudes. The continuous descent information usually comes from the host nation’s aviation authority (i.e., not Jeppesen), but the current chart does not have these values. I do not know where Jeppesen got the 3.30 degree constant descent angle, unless they calculated it. That is permissible and is what must happen for the procedure to be coded into an FMS.


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Exhaust temperature of CFM56 engines range (Nozzle downstream temp)?
EGT is available but not the temp of the exhaust nozzle in the CFM56 series.

EGT is T49.5. Which engineering station are you looking for? T8? T9? Which CFM56 model?

@mins Are you adhering to the AS755 standard (Aircraft Propulsion System Performance Station Designation and Nomenclature)? Please do, we have created these  standards to communicate clearly about positions in the gas path (although sometimes even OEMs don't). Note that 3 digit (or 4) station numbering is now common, 49.5 is located in the turbomachinery according to the standard numbering (while it should have been 0495), the EGT starts after the last turbine exit (mechanical expansion process) which is station 050.

@Setare how far downstream are you looking at?

Turbofan engines CFM56 series.

My question is the Temperature downstream of the LPT and exhaust Temp from the nozzle

@0scar: I'm using AS 755, but made a mistake, it's not T8-T9 (I was in the afterburner!), but T5-T8 as you use in your answer.

The total temperature, denoted as Tt, remains constant throughout the nozzle as it performs no thermodynamic work. In the context of station numbering, the entry point of the nozzle is labeled as station 5 (or 050 according the AS755 Aircraft Propulsion System Performance Station Designation and Nomenclature standard), while its exit point is designated as station 8 (or better 080). Consequently, the ratio of the total temperature at station 080 (Tt080) to that at station 050 (Tt050) is equal to 1.0.
$$\frac{Tt_{080}}{Tt_{050}} = 1$$


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How can I get 8050-3 Registration amended?
How can I get 8050-3 Registration amended? Box 2 (Aircraft Manufacturer and Model) on 8050-1 Application shows 'MylastName, MyFirstName' and RV 9A as the Model.  When I got my 8050-3 Registration back, the box 'Manufacturer and Manufacturer's Designation of Aircraft' says: VANS AIRCRAFT, INC RV-9A which does not match Manufacturer (Me) on 8050-3 Registration.  This could be a problem when I apply for my Repairman's Certificate where I must be the manufacturer.  I tried contacting FAA.  NG by phone.   Sent Email.  No answer yet.

Paragraph 19 of the document entitled "INFORMATION TO AID IN THE REGISTRATION OF U.S. CIVIL AIRCRAFT" published by the FAA's Aircraft Registration Branch states:

CONVEYANCES SUBMITTED FOR THE RECORD

AMENDMENT. An amendment (such as to the date, the description of the collateral, the title of the signer, names of the parties, etc.)
may be submitted to amend, correct or clarify any instrument which is
on file with the Aircraft Registration Branch. The amendment should
state its purpose and describe the original instrument by date,
parties, collateral, and, if recorded, the FAA recording number and
date. The amendment must be signed by the parties to the original
conveyance. The recording fee is $5 for each item of collateral
affected.

The document states the mailing address to where you send the amendment request.


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How was altitude calculated before the invention of the altimeter?
I just read David McCollough's "The Wright Brothers," and was surprised to find several references to the exact altitude attained by various Wright airplanes.  For example, on page 235:

The next day came news from Potsdam, Germany, that Orville had flown to an altitude of 984 feet, higher than anyone had yet flown in an airplane.

The flight in question occurred in 1909.  But the altimeter would not be invented until 1928.  How can anyone be certain that such a specific altitude was reached?  I don't see any instrumentation in the photos of their aircraft.  Was the altitude calculated from the ground somehow?

While the barometric altitude was invented on that year, the way how altitude could be measured using a baometer has already been known for centuries.

Kollsman's invention was the mechanism to accurately calculate and display altitude from barometric pressure, adjust for ground-level pressure, and temperature-compensate the aneroid. Pressure altimetry was already written into regulations by 1919.

By triangulation. Knowing where the airplane is above the ground and knowing what the angle is between horizontal and the sightline vector pointing towards the plane, you can solve for the height of the resulting triangle and hence the altitude of the plane.

Ok, I get that.  And I get how you could use trigonometry to measure the height of a tree.  But how would an observer be able to know exactly what point the plane was directly above?

Or you could just use a barometer, which is all an altimeter is.  But yeah they could also use a sextant and observe the plane's angle from a known distance flying over a specific point.

@John K it would be optimistic to claim a resolution of 1ft even with a modern-day altimeter

Even using triangulation using a sextant, (see my revised comment) you'd be lucky to get an accuracy of better than 10 ft.

This is a classic translation problem. 984 feet is 299.9 meters. I'm willing to bet the original report was 300 meters. Which got translated into 984 feet. As 300 meters is a nice round number, that could have been a proxy for "299 - 301", "295 - 305", etc. or even "275 meters to 325 meters" - i.e., a range easily determined by triangulation with the tools of the time. In fact, if the measurement had been in the US, and they thought it was "984 feet plus or minus a bit" they would have rounded to a nice big 1,000 feet and then the foreign press would have reported 304 or 305 meters.

@nuggethead you use two measurement points simultaneously, with a surveyed relation to each other.

@nuggethead follwing up on fectin's comment, this is just ow GPS works, except GPS needs three points instead of two.

You would have to have a way of recording an angle instantaneously, and establishing that two measurements were made simultaneously. Those seem highly nontrivial to me, especially with early 20th century technology.

@manassehkatz-Moving2Codidact There's a similar issue with people referring to "normal" body temperature as being 98.6. It was originally only two significant figures (37) in Celsius, but it turned into three significant figured when converted to Fahrenheit (and even 37 is overly precise, as "normal" body temperature varies from person to person, by time of day, etc. etc.).

@Acccumulation: Even worse, Fahrenheit degrees are smaller (closer together) than Celsius degrees, amplifying the problem of introducing excess precision.  So it's about 18x more precise, not just 10x.  37+-1 C is like 98.6 +- 1.8 F.  (Some would argue that you should round 98.6 to 99, moving the center of your uncertainty range due to rounding error as well as the original uncertainty / variability, although presumably 37 wasn't right in the middle of typical range either.)  Anyway yeah, naivety about sig figs is widespread, but multiple roundings can accumulate more error.

@Accumulation: it only requires each observer to have an assistant with a reasonably accurate watch, which are synchronized before and after the experiment. They track the plane through their theodolite sights and read out numbers every few seconds like "Now: three-zero-six. Now: three-one-four.", while the assistant looks at their watch and writes down the time of each "now", and the number. When the timestamps of the two observers differ, you interpolate between them. I've been the assistant in somewhat similar measurements.

Note that they would long have had access to the techniques needed to do this from measuring the heights of tall buildings.  This was probably also used to measure the heights of lighter-than-air vehicles (balloons), which predate the Wright Bros. by a few centuries.

@Acccumulation For what it's worth, Wunderlich's original measurement of average body temperature was to three significant figures. He reported a mean oral temperature of specifically 37.0 degrees Celsius in his 1868 book (with a mean range of 36.2 to 37.5 among healthy adults when averaged by hour rather than overall). The point-zero just normally (and somewhat inappropriately) gets elided.

authors: Vladimir F Героям слава


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Why don't airliners use engines one behind another?
Why do airliners use the engines the way they currently do and not one behind another?? Would it not lead to better thrust and in case any one of the 2 engines placed together has to fail, the 2nd one will help keep it moving and with the fuel, we wouldnt completely lose all the thrust the engine used to provide.
What is wrong with my idea?

I think this is essentially an afterburner. After the air goes through the first engine the oxygen level in the air is low so the efficiency of the second engine is lower

Thanks @Wyatt I did not think of that point :)

@Wyatt, except If your engine fails you can't just turn on the afterburner. It's part of the engine, not a separate backup engine.  It relies on all the sections in front of it operating properly.

@MichaelHall Yeah that’s true. The second engine would still receive hotter and less oxygenated air, making it less efficient. (Given the first one is on)

Duplicate of What would happen if put a jet engine behind another jet engine?

There's already tremendous expense & engineering in building the hot section of a jet engine. Imagine having to build the entire "back" engine out of these materials, since they're ingesting hot exhaust gas instead of the cold (-30 - -50°C) air of the upper atmosphere.

Most airliners already have 2 engines, and can operate on one just fine.
Your suggestion would be very costly, adding massive weight and complexity, and burning way more fuel for additional engines that would be operating at reduced efficiency due to breathing in the oxygen depleted exhaust of the engine in front of it.
There are four engined airplanes out there, but they are largely going away due to the performance and reliability of modern twins. And if for some reason you did want to design a new four-engined airplane, you would probably want to configure them side by side and not in line as you suggest.
In short, it would create way more problems than it would solve.

I’m also imagining that if the first engine fails, it’s likely to lose parts, which will be caught by the second.

Depends on the cause... failed high pressure fuel pump, no. If birds FOD out the forward engine, then they & broken fan/turbine blades all go directly into the rear engine, and immediate dual-engine failure.

It wouldn't increase thrust.
Thrust means the gases coming out the back of the engine are moving faster than the gases coming in the front.  The bigger the speed difference, the more the thrust.
So if you speed up the air coming in (by feeding it the fast exhaust from another engine), then you're reducing its thrust.
A more precise explanation is offered by our beloved Peter, of course.

If one looks at the interior of a jet engine we do see many compressor and turbine blades one behind another.
With jets, it is most efficient to have one place where incoming air is heated to drive the turbine blades, which drive the compressor (and fans).
Indeed, the trend has been from many smaller jet engines to 2 larger jet engines.
Theoretically, the 2 could be replaced by one even larger single engine (imagine a 747 with a giant fan jet mounted on the back just like a Heinkel 162, but 2 engines provide safety in redundancy).
The English Electric Lightning did have 2 jet engines mounted one on top of another, rather than side by side, but no advantage is gained by having one jet behind another.

This is an easy physics. We don't need to think about engineering.
Jet engines work by compressing air, adding heat through combustion, and then using the hot, expanding gases to generate thrust.  Placing one engine directly behind another would expose the rear engine to the already hot exhaust of the front engine. This would reduce the efficiency of the rear engine because it would compress air less efficiently (reduced air density due to preheating).

This question is a bit like asking why car designers don't replace V8 engines with a pair of daisy-chained 4 cylinder engines. Whatever two separate engines connected inline can do, will almost certainly be done better when they are integrated into a single engine. This is why there are zero racing sports cars with dual engines in them. The closest thing I can think of is the Bugatti Veyron W16, which is like two V8s attached together. However, this is still a single engine, rather than two separate V8s connected together.
When you consider that a jet engine has to accelerate the air coming into it to actually provide thrust, it becomes clear that the outgoing air from one engine does not make for good incoming air for the other. The exception is if the front engine could accelerate the exhaust to high supersonic speeds, enabling the rear engine to operate as a [sc]ramjet. Unfortunately, a turbojet cannot accelerate air from subsonic to multiple mach numbers, but even if it could, the mass of the exhaust stream would have to be very small because the incoming air volume is not large enough to support such a large multiple of output velocity (the average flow rate between the input and output of the engine must be the same + fuel mass).
But even if you could design a turbojet with this multiplicative capability, there is still the question of why you would then use two separate engines rather than combining them into the same engine. In this case, all the hard work goes into the front engine, so there is no obvious benefit to adding the back engine at all.


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What does "ceo" stand for in Airbus A320ceo?
What does "ceo" stand for in Airbus A320ceo??

It means "Safety tests be damned, the BOSS told me to ship it!"

"In December 2010, Airbus announced the re-engined A320neo (new engine option), which entered service with Lufthansa in January 2016. With more efficient turbofans and improvements including sharklets, it offers up to 15% better fuel economy. The previous A320 generation is now called A320ceo (current engine option)."
Source Wikipedia

what's next A320ueo (updated engine option)?


text:
How can the wing generate more lift than the thrust?
I recently checked about the max takeoff weight of an A380 and the thrust it can provide at TO/GA power and I was quite shocked by the results. The total thrust by the engines was only about 127000 kgs whereas its max takeoff weight was 560,000, almost 4x the thrust by engines.

How does this make sense?
If we increase the wingspan, can we get more lift? And if that is the case, then
Can flying an airplane with a small motor with extremely long wings is possible?

Sure, will do from next time @sophit

If this wasn’t possible then why would anyone bother to fit wings at all?

If thrust were larger than weight, airliners could take off vertically, like rockets. That they cannot amply demonstrates that thrust is only a fraction of weight.

@PeterKämpf : in fact very few fixed-wing aircraft are capable of doing that. There are only a few acrobatic aircraft and fighter jets which can do it.

Wow, a duplicate of a duplicate!  Cool.  Maybe someone will ask a question that gets identified as a duplicate of this question and then we'll have a triple-duplicate?

A wing's shape allows it to deflect a large volume of air downwards while incurring a small amount of drag, which in turn means a relatively small amount of thrust (compared to the weight of the plane) is required to push it forwards. The critical value involved is the lift-to-drag ratio, which varies based on a number of factors but might be 60:1 for gliders or 20:1 for airliners.1
Perhaps an intuitive way to understand how this is possible is with a diagram. In this simplified representation, the wing is the gray box and it's moving to the right, so the blue arrows represent the direction the air is flowing relative to it.

As the wing moves through the air at an angle, it will produce a force perpendicular to itself (the yellow arrow). At low angles of attack (that is, the angle between the direction the wing is pointing and the direction the plane is moving), you can see that the vertical component (lift, colored green) is much greater than the horizontal component (lift-induced drag, red). There some additional "parasitic" drag created even if the wing is pointing straight into the wing (orange arrow), but even if you add this onto the induced drag, the lift is still much greater than the total drag.
Now for your second question: If you increase the wingspan (and in turn wing area), you will increase the lift at a given airspeed, but you will also increase the drag as there is now more wing area that has to be pushed through the air. This will reduce the top speed of the aircraft but will probably allow it to fly at a lower airspeed. The ratio of lift to drag actually will improve slightly due to what happens when aspect ratio (the ratio of wingspan to the distance from front to back of the wing)
is increased - see here for a detailed explanation. This is the reason for gliders having extremely skinny wings.
Answering your third question is tricky, as it depends on a number of factors, but my intuition says increasing the wingspan should result in less power required to get off the ground, up to a point. Longer wings are weaker, so they will need more structural weight to support them. There will come a point where the extra lift generated by adding more wingspan will not be worth the weight and drag caused. I suggest an optimized design would be something along the lines of human-powered aircraft like the Gossamer Albatross, as these have to work around the low power-to-weight ratio of humans.

1. These numbers are actually lift-to-drag ratios for the whole plane (so including the drag from the body), so the lift-to-drag ratio of the wing itself is actually a little bit better. The ratio for the whole plane is what matters when trying to calculate how much thrust it needs to make a certain amount of lift, so that's why I've used it here.

Just needed to confirm one thing though. The engines are just there to counteract drag?(And get off the ground)

@SambhavKhandelwal When the plane is cruising at a constant speed: yes, the engines are only counteracting drag (because drag increases the faster you go, if you leave the engines at a constant power the plane will end up at the perfect speed where drag matches thrust). When the plane is accelerating such as during takeoff, the engines are also working to increase its speed obviously.

@ThatCoolCoder Small corrections: I'd use the following picture (just an example) instead since it better represents how air encircles the airfoil and doesn't just bump into it (something true only at hypersonic speeds). "Now for your second question: If you increase the wingspan, you will increase the lift" I think you wanted to say "Now for your second question: If you increase the wing area, you will increase the lift"

Lift is produced when the wings push air down. Doing so inevitably involves some drag that would tend to slow the airplane down or force it to lose altitude. The engine only needs to provide enough thrust to counteract drag, which is generally several times less than the lift.
Yes, flying airplanes with small motors and extremely long wings is possible. The downside is that these airplanes tend to be slow. Some extreme examples are the MIT Daedalus (where the "engine" is a person) and Solar Impulse (a solar plane with a wingspan similar to an A340, but some 100x lighter).
Some people even fly gliders, essentially airplanes with no engine at all, for long distances!

Just to complement a bit the other answers.

How can the wing generate more lift than the thrust?

The question is based on the false premises that:

lift is always much bigger than thrust; and
lift and thrust are directly correlated.

Regarding 1.:
Lift can be bigger than thrust but it doesn't have to. The ratio 1:4 that you've calculated is absolutely no fixed value and it's related to the type of mission that the airplane has to fly more than to the physics behind. A value 1:4 till 1:5 is quite typical for a jetliner but a value 1:1 (or even higher - more than 1.25:1 can be reached by a F-22 for example) is more typical for a jetfighter: for a jetfighter your original question is therefore just the other way around. And a glider doesn't even have a proper thrust source so that that ratio just become infinite.

Regarding 2.:

The total thrust by the engines was only about 127000 kgs whereas its max takeoff weight was 560,000, almost 4x the thrust by engines. How does this make sense?

This totally makes sense if you consider how the four main forces act upon an airplane (picture from this question):

As you can see, thrust wins drag and lift wins weight and in general for an airplane there is no direct coupling between lift and thrust: if your airplane had twice as much drag, then the thrust generated by the engine should simply be twice as much but weight and lift wouldn't change. Or, if your airplane were 20% lighter then the lift should simply be 20% smaller but drag and thrust wouldn't change.
Obviously the whole story is not plain black and white as just described: in my first example, if drag doubles then thrust must double, then the engines become bigger and heavier, then weight increases, then lift increases, then drag (due to lift) increases and then we've just entered in a vicious circle. Anyway the general idea that weight and thrust are acting in two different directions holds true.

You are able to pull a sled or a cart way heavier than yourself, uphill, pulling it with a force lower than its weigth. Of course, if the path is way too much inclined, you will fail.
It is the inclined path that helps. I will skip the geometry pictures here as everyone has the intuitive understanding on the matter.
In the aviation context, it is the pilot who decides the inclination of the climb by controlling the pitch angle (how much the nose is up or down) and the control surfaces positions.
At a speed, the air under the wing behaves as a snow surface that is generally as much inclined as the wing is. Happy climbing!
The airplane behaves as a sled when going downhill as well, it gains speed and one has to deal with the unwanted speed in order to, e.g. land safely.


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Do altimeters still work in extreme angles of flight?
I've heard that the way the plane measures its altitude is to use a barometer to measure air pressure.
Does the pressure at the static ports drop as the aircraft's speed increases?
There is no perfect way to get true static pressure, an answer in the above question has showed that the pressure measured is different along the fuselage. So static pressure error correction(SSEC) must be done to measure true altitude. I think its in a form of lookup table of (angle of attack, angle of slip, gyro angular velocity)=>(SSEC value).
But this lookup table is too hard to build, so I guess, for cost considerations, altimeters are only designed to work in normal flight. When the plane enters spin, the altimeter reading will jump rapidly randomly. Fighter pilots will also see altimeter jumping when they are in dog fights. Is that right?

Some aircraft have multiple static ports.  For example the Grob-103 has at least 2 and possibly 3 or 4.  This reduces the error you mentioned in various situations, including during sideslip.

However, it is difficult to prove that the errors can be perfectly compensated by multiple static ports at any fly angle. I guess accurate height is not that important in extreme angles, for example when you enter a spin, you are not taught to watch the altimeter to address the problem. Dog fighters also don't watch altimeter in a fight, they watch their relative height to enemies. When you approach and land, the altimeter will work again and you can use it to help you land.

If you're in a spin you're supposed to be focusing on getting out of it, not staring at the altimeter to see what it is doing.


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McDonnell Douglas DC10 - scale model
I am building a scale model of a DC10. I need precise details of the external lighting arrangements. The red and green wingtip nav lights are obvious but I need the details of other beacons and their precise locations - red flashing dorsal and ventral and any white lights - flashing or steady.
I would be grateful for any help you can give.

Airport planning docs sometimes have this information, check here https://www.boeing.com/resources/boeingdotcom/commercial/airports/acaps/dc10.pdf
I seem to recall an extensive website dedicated to the DC-10/KC-10 with a huge amount of technical detail.  Unfortunately, I can't find it at this time.

This might help

There is some details in this online document. Not much details but as you should be able to fix the location fairly accurately.
I seem to remember the landing lights only swing out when activated and remain flush otherwise. When activated the lights will switch on and swing out so when you observe them as they deploy the beam will point sideways and swivel until they are pointing forwards.
https://dokumen.tips/documents/mcdonnell-douglas-dc-10-flight-crew-operating-manual.html?page=10


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What are these clip-like things?
With me becoming an aviation enthusiast and AirBerlin becoming insolvent, my mom has gone on a hunt for deals on what I can best describe as AirBerlin merch (old uniforms, shirts, skirts, belts and many more).
In one of the packets that arrived recently she found items that neither her nor I knew the purpose of.
They are about 2cm * 2cm, yellow, have some kind of number on them and on most of them was the writing "Air-Berlin SEALED" or "airberlin group SEALED" and an engraved "PULL". It also looks like you can break off and bend a part in the plastic to the back and clip it there.
Below is a picture I took of a few of these:

Galley seal. It works like a cable tie, you can put one end inside a hole on the main body but you can't pull it out without destroying the seal.
The cabin crew will seal the galley carts containing alcohol and other duty-free items before landing and note the seal numbers in their report. This is required by law in a lot of countries.

That makes sense! Do you have an image of one of these in their designated place by any chance?

@geisterfurz007 Admittedly, this is not from AirBerlin, but in this picture you can see a seal on a cart. Different airlines use different seals, naturally.

Oh yeah, I meant galley seals in general, not only AirBerlin; thanks for the picture!

Do you know any way to find more information about the laws that require this? Keywords to search with or case numbers etc

@cat seals like this aren't limited to the drinks cart on airlines - basically, any inventoried/counted stuff left in a rack/tray/etc. could use it.  Hospital I worked at we used them all the time on crash carts to ensure everything was there and proper, to close/seal/latch large tubs of equipment to designate them as being "clean and ready for patient use", etc.  They basically provide an easy accounting/control method that has something almost like an audit trail on it...

Electric utilities in the US us something similar. Since it can't be opened without destroying the seal, but destroying the seal is easy, it's not meant as security, simply as a means of tamper identification.

Oh indeed! A search for "crash car seal" bring up the ones that look like the bottom one in my picture. Great hint!

Of course, all one has to do to make the tampering undetectable is to carry a seal with them to reseal whatever they're tampering with after they're done tampering with it.

The seals are numbered and sometimes this will help identify if the seal has been broken. Eg when someone seals it, the seal number is usually recorded and simlarly at the other end when the cart is opened. If any discrepancy is noted the seal numbers will be compared.

Not fool proof but it works for the most part.  It's usually required to be sealed for different conditions. Eg some countries require all liquor cart to be sealed before landing. Duty free carts are also sealed when the flight is completed to prevent any items being stolen. Its not perfect but there are several safeguards in place. For instance seals are color coded to departments.. the flight kitchen will use one colour, cabin crew and security will use others.

Also the seals are numbered and it is possible to trace the seals as they should be recorded when issued to a person. Having said that it all depends on discpline as I've seen them lying around in the galleys.


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how does the shape of the fuselage effect aerodynamics?
For example the shape of a fighter jet compared to a boeing 747.

Hi.  Welcome to Aviation.SE.  This question is too broad to answer within the format.  There are many factors e.g. payload, speed, range, cruising altitude, usage, legacy from previous designs, economics etc etc etc. Please have a look at the help centre.

You may narrow you question to one really specific aspect (e.g.  wings design, engine location, ...)

as it is said in the comment, there is no short answer for this one.
If we just focus on the Aerodynamics, there are some important differences between the approach used in commercial and military aircrafts when designing an aircraft. I believe the most relevant are the following.
Firstly, commercial planes are designed for an optimal cruise condition in order to reduce fuel consumption. This is function of the Mach, the performance of the engine and the aerodynamics of the plane. On the other hand, for a fighter as the one you mention, good performance has to be achieved for an entire envelope, including subsonic-supersonic regimes and different altitudes. This will determine characteristics like the shape of the wing and fuselage itself, engines (very important to take into account for the aerodynamics as well)...
Secondly, in terms of stability, you will find opposite approaches. It is very important to understand that one always has to sacrifice stability in order to achieve some levels of manoeuvrability.  A commercial plane aims to achieve high levels of longitudinal and lateral stability; whereas a fighter needs to be able to perform way more demanding manoeuvres and is inherently unstable. This will of course be an important part when designing a fighter and set the aerodynamic centre of pressure (size of lifting surfaces, fuselage, use of configurations such as a canard…).
Again, these are some factors that are taken into account, however I hope this helps to understand some of the differences from the aerodynamics point of view.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


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XRotor blade angle distribution
My question is about the XRotor definition of the blade angle distribution (beta), which XRotor solves for or requires as input. Is the blade angle on XRotor defined as the angle between the rotation plane and the chord line of the blade section (geometric pitch angle) or is it defined as the angle between the rotation plane and the zero-lift line of the blade section?

This is a question about a particular software application, not about aviation in general. Better to see what support the software publisher makes available.

The normal definition of the blade pitch angle is between the rotation plane and the chordline of the blade section. It's a geometrical feature (so not dependent on the lift of the blade).

I'm pretty sure it is relative to the foil chord line.
You should be able to do some tests to confirm this.


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Function of the compressor in a gas turbine engine
I am trying to understand the importance of having compressed air in the combustion chamber of a gas turbine engine. As far as I understand it, the reasons are twofold.
Firstly, the compressor is necessary for the engine to produce static thrust, as the pressure gradient gives the engine a "preferred" direction of airflow.
Secondly, the compressed air in the combustion chamber results in more molecules of oxygen resulting in more efficient combustion - the same reason why superchargers and turbochargers exist for ICEs.
Is there another reason - potentially thermodynamic - that I am missing? Like how an ICE requires a compression stroke for the power stroke to be able to occur, is there any fundamental reason why the jet engine requires the compressor? Would a jet engine need the compressor (ignoring air density) when the aircraft is at speed?
Thanks.

Please put your answers in a real answer, not in the comments.

Would https://en.wikipedia.org/wiki/Ramjet be an answer to your question, or are you looking for an possible engine where no compression occurs at all (regardless if powered by a shaft or air inertia)?

The ramjet has no compressor, only ram air is used, see Is the compressor required on jet engines? Can air be rammed into the turbine?. This is possible only after Mach 2 or 3, at this speed the required quantity of air can be rammed, mixed with fuel at the correct air/fuel ratio, and can deliver the required energy/thrust. So yes the first reason of the compressor is to feed the combustion chamber.

@Martin , yeah I should have clarified, excluding ramjets. The idea essentially was why a continuous pulsejet (a contradiction, I know) doesn't exist.

Don't you think that first reason, 'to produce static thrust' describes a result, not an actual 'reason'?

To add to all these great responses, the ideal gas equation shows us PV = nRT (T is in Kelvin).  For our isobaric turbine, we have V proportional to nRT.  We gain volume by increasing T, but also by increasing "n" or density of the gas.  So the compression x temperature increase yield higher volume, with further benefits from more efficient combustion. von Ohain's first turbine failed with petrol, but ran well enough on hydrogen to continue progress.

@RobertDiGiovanni am I misunderstanding or should you mean concentration, not density?

@bchen same thing: molecules of air per volume.  (Air composition is usually the same).  But you make a good point: it could be different.  This is why humidity can affect aircraft performance.  Water molecules are lighter.

@RobertDiGiovanni interesting, would the main performance difference from humity be from engine efficiency or other factors?

@bchen the m in mv.  Water molecules are 18, N2 is 28, O2 is 32.  But adding liquid water to make steam increases volume greatly.  It has been used to boost take-off thrust.

You don't. A (sc)ramjet doesn't have a compressor, as the forward velocity of the jet provides the necessary pressure.
Most thermal engines do work by expanding a gas. In many cases, this gas expands against a piston, and the piston converts the heat energy into mechanical work. A turbine does not contain a piston to expand against, and the walls of the turbine offer neither movement as required by a piston, nor thrust even if they could move. Thus, the combusting fuel must expand out the front and the back of the engine. Obviously, expanding out the front is undesirable, as that provides negative thrust. Nor is it feasible to place a piston there for the gas to expand against. The solution is to create a virtual piston out of air, by squeezing incoming air so that when the combusting fuel expands, its only choice is to expand out the rear of the engine. The fact that this also improves oxidizer flow is a virtuous benefit.
When the fuel combusts, it increases the pressure at the site of combustion. This pressure wave expands spherically, because it doesn't "know" which direction it is "supposed" to go. The walls of the turbine confine the pressure wave in the radial direction, so all of the pressure must travel axially, both forwards and backwards. If the pressure in the front of the turbine is lower than the pressure created by combustion, then some of the exhaust will flow forwards through the engine, which I think we can all agree is not the desired behavior. The only way to prevent this forward flow is to ensure that the pressure in front of the combustion chamber is at least as high as the pressure created by combustion itself. And that establishes the minimum pressure you must generate just to make the combustion products flow strictly out the back of the engine.
So yes, the compressor is essential to guarantee that the exhaust only goes out the back of the engine, and not the front. And yes, the compressor helps improve oxidation of the fuel, just like a super/turbocharger in a piston engine (and why the original jet engines were called "turbojets"). And yes, forward movement of the engine is sufficient to provide compression, provided adequate airspeed.
Although a ramjet can theoretically operate at speeds as low as Mach 0.5, they are typically considered optimal between Mach 3 and Mach 6.

The compressor of a ramjet is the combination of speed and intake - remove one or both and it ceases to produce thrust. The compression step is necessary for all heat engines. Naming the part that covers this step "compressor" in only one type of them does not mean that all the others can live without the compression step.

I'm confused. The first sentence of this answer is "You don't [need a compressor]." Then three paragraphs later it says, "So yes, the compressor is essential..." Which is it?

@ToddWilcox Is it not clear that only ramjets can function without a physical compressor?

It is not clear in the answer, no. Also, it's not clear what you're saying is the etymology of "turbojet" is. My understanding is that the "turbo" prefix refers to the use of a turbine. So a turbocharger is a turbine driven supercharger (forced induction device/air compressor). A turbojet engine is a jet with a turbine driven compression stage. Etc.

The reason is thermodynamics: without the compressor, the power output of a gas turbine would be exactly zero. It's not about being "more efficient" but about fundamentally being able to produce any power at all.
All heat engines work on the same principle: raise the pressure of a working fluid (gas) to a higher pressure and, and then lower the pressure again. In itself that's a futile exercise, unless you can somehow get more work out of lowering the pressure than you put in raising the pressure. That's why we do the combustion in the middle: you keep the pressure the same* but you just get "more" gas volume (it expands because it's hot) so you can get more work out of it than you put in initially.
If you didn't raise the pressure initially, you'd just have more, hot gas at ambient pressure, but it'd be useless unless you had a hot air balloon to fill.
*A combustor in a gas turbine cycle (Brayton cycle) is necessarily approximately isobaric (constant pressure) because it is open-ended on both sides. Other types of engines have different conditions during combustion; e.g., an ideal Otto cycle (petrol engine) is isochoric (constant volume) during combustion, resulting in an increase in pressure. This means that in theory, an Otto cycle doesn't need the compression stroke, leading to a degraded efficiency Lenoir cycle of a pulsejet.

Thanks for the answer. I'm still left wondering why raising the pressure is a necessary preliminary step. Is the pressure gradient analogous to the hot air balloon, providing a framework in which the expansion of the gas does useful work? Again, in an ICE it's easy to pass compression off as a requirement for efficient combustion, but it is clear that complete combustion is not the only effect at play here.

You are right: thermodynamically you need a "raise the pressure" step but it doesn't have to be a compressor. Like how a rocket uses the energy of burning fuel directly to raise the pressure and propel gas out the back. But a rocket has a closed front whereas a gas turbine needs to draw new air in to combust. And since a combustor with two open ends is largely isobaric you need the compressor.

An interesting case for comparison, intermediate between a turbine and a rocket  is the ramjet in which the compressor is essentially just a nozzle, using the forward motion of the aircraft, missile etc. to compress the incoming air, and thus unable to run without forwad movement

I don't understand your answer at all. The energy - the pressure raise - comes from burning fuel. An ICE does not need a compressor, why does a turbine engine?

A hot air balloon trip flips this cycle on it's head: expand hot air, cool the air, then compress the cooler air during the descent. This (inefficiently) converts heat energy into work raising the balloon.

@VioletGiraffe My point is that you need a high pressure somewhere in the cycle to do any work. An ICE could just close a valve and get some very inefficient work out. A gas turbine combustor doesn't have valves so it needs some other way to make a part of the cycle at a higher pressure. A compressor is the typical choice.

@VioletGiraffe Well, in order for a jet engine to work, air has to flow into the front of it. If you use combustion to create a high-pressure region, and you don't have a compressor, then air will stop flowing into that region (unless you're going fast enough for the dynamic pressure to overcome that static pressure), and your engine will stop working.

The compressor raises the pressure and temperature at which the fuel is burned, thereby increasing the thermodynamic efficiency of the cycle (Brayton) on which the jet operates.
Second, without the compressor stage in place, the combustion gases would be free to blow out the front of the engine as well as the exhaust, thereby generating no thrust.
The majority of the power output of the engine is used to operate the compressor. The remainder is used for propulsion.

Thanks Niels, this answers the original question.
It is curious that the majority of the power output operates the compressor. I assume that the power comes "extracting" the kinetic energy of the jet. Are there any measures in place to ensure that the momentum of the exhaust jet is relatively unaffected? That is to say - is the turbine designed such that the thrust produced is similar?

The turbine is designed to extract just enough energy from the exhaust to spin the compressor, or the compressor plus bypass fan. This sets the number of turbine discs, their diameter, blade pitch, and number of blades per disc.  The remaining thrust is set by the mass flow rate  times the pressure drop across the turbine discs.

I am intrigued by the last sentence: I would have thought thrust would have related to the pressure drop between the free stream and the exhaust jet, as T = mdot * v?

Whilst it is true that the compressor consumes a lot of power, I don't think it is correct to contrast it with the power for propulsion. The compressor is a major contributor to the propulsive force, and thus to the propulsive power.

I thought the majority of a turbine engine's power is used to drive a propeller or a bypass fan?

I am not a specialist, but it seems to me the energy used by the compressor goes into air pressure which is then used for propulsion, so it is not lost.

@ florian f,  study the p-v diagram for the brayton cycle.

In order for a jet to create thrust, pressure must be created so the difference in pressure from the inside of the engine to the outside air will create a flow of a certain mass at a certain velocity.
This mv, or momentum, will drive the engine (and aircraft) in the opposite direction.  We've all done this with a balloon, blowing it up and letting it go.

a "jet engine" is only one way of doing this

The aeolipile used heat to raise steam pressure.  Rockets pump liquid oxygen and fuel into their combustion chambers, creating pressure by both phase change and temperature increase.

the name of the game is constant mass flow

As Lawnmower Man eloquently put it, we are creating a virtual piston, and the compressor is a virtual intake valve, continuously feeding air into the combustion chamber as heated air is exhausted.

because the incoming air is heated, the exhaust port will be larger than the inlet.

A bigger hole in back than in front.  Going back to rockets, they must carry their own oxidizer.  Breathing air with a compressor saves carrying all that extra weight.
Out of that bigger hole in the back comes thrust.  The heat energy added into the system is also converted into mechanical energy to drive the turbine (which drives the compressor).

Interesting observation that the compressor is a virtual intake valve. To take the analogy further, I assume that the combustion chamber acts as the compression stroke, with some blurring of the lines between the power and exhaust strokes shared between the turbine and exhaust nozzle?

@bchen exactly.  The jet is continuous.

Breathing air with a compressor saves carrying all that extra weight. - And even more importantly, the atmosphere gives you something to push on that you weren't carrying with you.  Momentum and thus thrust scale linearly with exhaust velocity, but kinetic energy scales with v^2.  So it's much more efficient to push a lot of air through an engine so the same amount of energy can create a lot more momentum from lower exhaust speeds.  (Turbofans take this a step farther and take energy from the jet to move more air with a big fan.)

Energy density of jet fuel vs. jet fuel + LO2 is what, maybe a factor of 2 or 3, that kind of ballpark?  But air-breathing turbojet or turbofan vs. pure rocket is what, something like a 20x difference in impulse (newton-seconds, which would be delta-v if there wasn't drag) per amount of fuel?  https://www.sciencedirect.com/topics/earth-and-planetary-sciences/air-breathing-engine#:~:text=However%2C%20the%20specific%20impulse%20of,higher%20at%20hypersonic "the specific impulse of an air breather can be an order of magnitude greater than rockets at the lower Mach numbers" (not hypersonic)

@PeterCordes actually not too surprised about the specific impulse comments. The GE-90 - an introduction and some nasa data states that the mass flow through the ge90 is "only" half that of the F1 engines on the saturn v first stage. Consider then that the rocket carries its own oxidiser and the v high jet velocities and it is understandable that rocket efficiency is so low in low and slow conditions.

@PeterCordes maybe we should be careful to not conflate the compressor and the intake fan - one (as I understand it now!) is thermodynamically necessary for the engine to be able to do work, and the other increases propulsive efficiency

Would a jet engine need the compressor?

All heat engines require a compression step, only in turbomachinery the part involved is called compressor. But other engines also compress a medium as part of the cycle:

In steam engines it is the feed-water pump that must overcome the boiler pressure to add more water to the boiler. By doing the compression work on a liquid medium, it does so very efficiently. You could call it also a compressor - the task is quite similar.
In piston engines it is the cylinder-piston combination during the upstroke that compresses the fuel-air mixture before it is burnt.
Liquid-fuel rockets use turbopumps to feed fuel and oxydizer into the rocket's combustion chamber. Given the mass flow of rockets, their energy density is impressive. They could as well be called compressors.
Even pulsejets use a compression step similar to a piston engine, only here the piston is replaced by an oscillating pressure wave.
and, lastly, ramjets use the combination of intake flow speed and intake geometry to compress their working medium. Remove one of them and they cease to work.

That only jet engines give the compressor its proper name is no reason to believe that its contribution is not essential to the work of heat engines in general. The why is excellently explained by both Sanchises and sophit, so I do not repeat it here again.
What leaves to be said: The compression ratio determines engine efficiency and mass-specific thrust. The Jumo 004B had a compression ratio of just 3.2:1 and had twice the thrust-specific fuel consumption of the equally sized EJ200 (without afterburner) with a compression ratio of 26:1, but the modern engine has seven times more thrust at approximately the same mass as the old Jumo engine. Higher compression is also the reason for the better fuel efficiency of diesel engines.

I think that both a practical and a thermodynamic answer can be given.

As far as I understand it, the reasons are twofold.

Correct
From a practical point of view, compressing the air before the combustion chamber allows:

the air to reach a temperature high enough to make the mixture air-gasoline self ignite; this is similar to any diesel engine that indeed does not need any spark plug;
the airflow to move from the compressor towards the turbine; the combustion chamber is opened at both ends and therefore the combustion products would naturally come out from both ends; having on one side of the chamber (the one towards the compressor) a much higher pressure than the other one (toward the turbine), forces the combustion products to flow towards the turbine; in a piston engine this job is done by the cylinder head.

Is there another reason - potentially thermodynamic - that I am missing?

From a thermodynamic point of view, the compression stage as well as the turbine stage are the two steps needed to extract work from the cycle:

In this picture (Wikipedia), the points:

1 to 2 represent the compressor stage; the compressor raises pressure and temperature and diminishes volume;
2 to 3 is the combustion chamber; being the combustion chamber open, pressure remains more or less constant and only volume and temperature of the combustion products increase;
3 to 4 is the turbine; it reduces pressure and temperature and the combustion products expand;
4 to 1 is the closing part of the cycle representing the fact that the exhaust gases return to their initial state.

The p-V diagram is quite useful since the area enclosed in the loop represents the work extracted from the cycle.
Now, what would happen if the compressor stage weren't there? Line 2-3 and 4-1 would overlap bringing the inside area to zero: no net work could be extracted from the engine which would simply behave like a furnace.

why can't the increase of pressure represented by 1-2 occur from combustion?

It definitely can occur in the combustion chamber and this is actually what happens in any Otto or Diesel engine: the combustion takes place in a fixed volume and therefore increases the pressure that pushes the piston down. But the combustion chamber of a jet engine cannot be confined, it must be open in order to continuously ingest fresh air from one end and expel the combustion products from the other end. The combustion happen at constant pressure (since the combustion chamber is open) but with increasing volume.

Really appreciated answer,and the PV diagram is very helpful in the understanding. Point of confusion which stems from the original question: why can't the increase of pressure represented by 1-2 occur from combustion? Since the majority of the work done by the engine originates from the fuel's chemical energy, why is the external pressure increase required? Thanks!

@bchen: simply because of all the practical reasons I've just listed. For the combustion to happen you need 1) a lot of air 2) at high temperature and 3) flowing from the inlet to the outlet of the engine. Compressing it is the way to get all of these 3 points. Plus (thermodynamic requirement) if the pressure at 2 and 3 would be like 1 then the turbine couldn't work either.

@bchen: is it a bit clearer now?

yes - it did take me a minute. I understand it as the thermodynamic pressure requirement is a "byproduct" of the practical considerations for the combustion to occur in the right way. Thank you very much!

@bchen: For me, understanding dawned when I considered a counterexample: extract work from combustion without compressing the input air before burning fuel mixed with it.  The combustion itself can raise the pressure if you confine it (like a rocket nozzle), which avoids the problem of an empty area on the P-v diagram.  But we wanted to design a continuous-combustion engine, not a pulse-jet, so we need to feed new air into the combustion chamber.  Which has to be at higher pressure than the free stream to generate thrust, so air won't move into it without some kind of pump, aka compressor.

@PeterCordes really helpful counterexample which links (?) the themodynamic side to the practical intuitively


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How are flaps and the elevator positioned in cruise?
so I understand that an elevator is used to maintain cruise (straight level and unaccelerated flight) but I’m curious now about how the flaps and elevator is positioned whilst an aeroplane is trying to maintain cruise?
Anyone know any good books on this sort of things please or a video maybe please?
Also why/how is it that a flap cannot be deflected in cruise and an elevator can? What’s the difference?

This NASA's guide should help

Hi thanks Sophit, I’ll take a look tonight

You're welcome. And please wait at least 24 hours before selecting an answer in order to give all the people around the globe a chance to come up with a proper answer

Wing flaps are used to significantly change the wing (size/shape/aerodynamics) so that it flies better at the slower speeds associated with taking off & landing. With rare exceptions (fly-by-wire F/A-18, for example), flaps are set in fixed positions and aren't moved continuously during flight. For cruise flight, including everything from about a minute or two after takeoff until a few minutes before landing, the flaps are in the fixed "up" (i.e. fully retracted) position.
The primary flight controls, the ailerons, elevators, and (to a somewhat lesser extent) rudder, move dynamically throughout the flight, controlled by the pilot or the autopilot, to maintain the desired attitude. If the aircraft starts to climb (when level flight is desired), the pilot applies forward pressure on the yoke, lowering the elevator very slightly, increasing lift on the tail and lowering the nose by the small amount needed to arrest the undesired climb.
These small corrections continue throughout the entire flight to keep, and restore, the aircraft's attitude to the desired attitude. Much like the small corrections required to keep an automobile centered in its lane while driving, even as curves in the road, winds, variable road surfaces, and the like tend to cause the car to drift left & right.
Edit: Flaps have limitations regarding how fast the aircraft is allowed to be flying when they're extended, since putting them out at high speed can impose stress on the flaps & the associated hardware that connects them to the wings. For airliners that cruise high enough that cruise speeds are measured in high Mach numbers (i.e flying around at a large fraction of the speed of sound, where transonic airflows can have effects on the flight controls), there can also be limitations on how high you're allowed to extend flaps. In the 737, those limits are 250 knots and 20,000' for the initial extension, and around 168 knots for full flaps. So in typical cruise flight, you would be out of parameters to extend them.
Happily, at cruise speeds, you don't need to extend them, since you're well outside of the low-speed regime where they are helpful.

Not to get overly detailed but I think it's worth also mentioning slats (perhaps this comment is sufficient), which serves the same general purpose as flaps but are located on the leading edge of the wing on most jet aircraft, at least all jet airliners seem to have them. Slats are normally extended  at the same time the flaps are. However they are often not noticed because of the much smaller size and shorter movements.

@StevePemberton  I think that's a level of detail better suited for a separate question

RalphJ - The OP likely didn't mention slats possibly because they weren't aware of their similar function to flaps, or possibly even their existence, but I think it fits into their question "how is it that a flap cannot be deflected in cruise and an elevator can. Since much of  your answer also applies to the slats I think it's worth at least mentioning their existence and general similarity to flaps to inform the OP. No other detail is needed, i.e. the technical differences between slats and flaps, which I agree would be better in a separate question. And maybe the comment was sufficient.

There's likely to be automatic cruise flaps, a.k.a. variable camber, in all new airliners; the 787, 777X, and A350 all have it.

Thank you for the help guys, especially @RalphJ!

Sorry I just have one more question please. So after the elevator is lowered to gain more lift, does the pilot then retract it back to normal position? Or leave it lowered down please?

@RalphJ, where you say "These small corrections …", you could also mention "trim". (This would also answer James's latest comment.)

@James While flaps are extended and retracted, the elevator simply pivots. The motion of the yoke is continuous, from full forward to full aft, corresponding to the full range of elevator travel. It's never retracted, just repositioned.

@RayButterworth Discussions of how elevators (or other flight controls) are trimmed would be better in their own question, rather than broadening this one any further.

@James, when driving a car, releasing the steering wheel allows it to go back to its natural position. Elevators behave the same way, but there is a trim control that allows this natural position to be slightly adjusted. Typically one flies at a steady speed and then adjusts the trim so that it maintains a constant altitude.

Hi again everyone! Sorry I’m still a bit confused @RayButterworth, but then if the elevator is not returned to like a non-deflected position please, wouldn’t the plane still climb? Do you suggest I open a new question for this please?

@RayButterworth - I tend to agree with Ralph J that trim is probably out of scope for the question if we assume that "how the flaps and elevator is positioned" is asking only what position the elevators are in, not how the elevator gets into those positions (cables, hydraulics, motors, trim, etc).

@StevePemberton, okay. But rather than a new question, James can look at
What does the term "trimming" most commonly mean in aviation?,
and for more details, look through all the questions tagged as "trim":
Newest 'trim' Questions.

@RayButterworth - agreed any questions about trim are possibly already covered in existing questions, that's the first resource before asking a new question.


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How is an airliner trimmed nose-down or up?
so when a plane is in cruise, it loses weight and then the weight begins to exceed lift.when this happens, the plane begins to climb.
My question is this please:
The plane would be trimmed up or down to maintain the straight flight, but how do you know when to use which one?
Also, wouldn’t it mean that if the elevator for example was kept in its new position, the plane would continuously climb? How do pilots stop this continuous climb?

Doesn't this answer to another question of yours answers your question already?

Hi, I looked through those and they don’t answer this question sadly. But I think I got it, can some some check my answer please?

Have a look to this basic NASA guide. There are also interactive simulations to understand how each control surface works.

Hi sophit, I’ll take a look now

In your different questions you're focusing on secondary elements (cruise phase, elevator trimming, fuel weight influence on altitude) trying to understand how altitude is controlled. But the main factors controlling altitude are lift coefficient, angle of attack and velocity. You should focus on understanding that first before entering into specifics like how to keep altitude constant or to trim the elevators.

I voted to close this question as it makes incorrect assumptions of how aircraft fly.

"it loses weight and then the weight begins to exceed lift.when this happens, the plane begins to climb.* I think you meant to say it loses weight and then the lift begins to exceed weight.

"My question is this please: The plane would be trimmed up or down to maintain the straight flight, but how do you know when to use which one?"  James, we are very forgiving of new guy questions here, but please think things through a bit before you ask.  May I presume you have driven a car or ridden a bicycle?  If you want to go straight but start to veer to the left, do you steer further left, or correct back to the right?  How do you know when to use which one?  Airplanes are no different.  I might suggest a familiarization flight with a local flight school to help understanding.

Trim affects the attitude of the aircraft in flight. To fly straight and level typically the aircraft would be in a slight nose-up attitude. If the aircraft had a good cg (slightly tail heavy on most types) you would not need much stab trim to acheive this. If the load-planning was screwed up and the aircraft is too nose/tail heavy, this would require more stab trim to get the aircraft level. More stab trim leads to more drag so will increase your burn-off. Hence the smart fellows at MDD (MD11) and Boeing (747-400) came up with the tail-tank as a means to correct the trim without using the stab trim (ie no drag penalty). Getting extra fuel capacity was a bonus.
To keep it level at different weights require different power settings, ie less thrust for xxx weight. Trim is not a factor.
There is also trim change due fuel burn off as the fuel cg shifts as the fuel is used up.. previously when doing manual loadsheets we would have a template that would mark out the cg curve.. interesting as the cg would move fore and aft as fuel was used.
On widebodies on long flights, initially it will be very minor changes in cg  as the fuel in the centre tank is used first. As this is in the centre section it has very insignificant impact on centre of gravity (ie not much trim correction required) but once wing tanks start supplying fuel, the cg changes as the fuel tanks are quite spread out due to the sweep of the wings.


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What is aeroplane trim and how does it control/adjust the elevator in cruise?
so in my last question, ‘trim’ popped up a lot and after reading up on it I got this: it’s used to adjust the forces on the control surfaces so that the aircraft maintains like a set input
But what I couldn’t find a simple answer on was this: how it controls/adjusts the elevator so that cruise is maintained even with the changing weight of the aircraft?
Also please, is the elevator left in the deflected position please when the plane gets the required lift to maintain cruise?

As mentioned by @Dan, trim is often achieved via a trim tab.  However, there are other ways.
On big jetliners, the entire horizontal tail can rotate to trim the aircraft.  The elevator (the control surface on the horizontal tail) is independently actuated as the primary pitch control system.  So in this case instead of the trimming surface being on the elevator, you could say that the elevator is on the trimming surface.
The important thing about a trim system is that after you adjust it, it stays put without continued effort.  This allows you to make occasional refined adjustments.
Trim appears in other similar contexts.  Some light aircraft use a metal tab on the rudder or aileron as a trim surface -- you literally bend it between flights to help balance out the aircraft for hands-free flying.
Boats can use trim both in the form of flaps on the boat to adjust its pitch attitude and also on the rudder for direction.

The trim is its own control surface on the back of the elevator called a trim tab.
The simplistic explanation is that its like a little mini 'wing' attached to the elevator that, when set properly, holds the elevator in the position you want.
To be clear, what I'm saying is that the trim tab is a) A physical thing, a part of the elevator and b) Is itself subject to aerodynamic forces. It uses these aerodynamic forces to expert forces onto the elevator.
Reference your questions, trim is delicate and will absolutely need adjusting as the flight parameters change. Its not something you set once and forget about it, it needs constant changes and a change in weight would definitely alter trim


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Why is it that when autopilot is maintaining cruise, we don’t feel the changes when lift increases?
When we’re inside an aeroplane, when we’re in cruise, why is it that we don’t feel the changes in altitude, lift and the adjustments for example from the elevator on the plane as the weight is burned in cruise?

Can you say what difference Question or exposition make between people and auto-pilots, other than that in theory, auto-pilots do things more smoothly?

People can feel even small changes in acceleration if they happen quickly, but slow changes in acceleration would need to be very large to be perceived at all.  While adjustments in flight parameters to deal with changing fuel load wouldn't affect acceleration, a plane in altitude-hold mode would nonetheless have a certain amount of undulating motion, and the perceptually smoothest autopilot algorithms would likely have larger variations in acceleration than an autopilot that was designed to minimize such variations.
Focusing on horizontal acceleration because it's a bit easier to reason about, imagine that one had a ball hanging on a 1m string from the ceiling of a plane that was accelerating forward with an acceleration of 0.01g.  That ball would seek equilibrium point that was about 1cm away from vertical.  If the plane's acceleration were to instantly drop to zero, the equilibrium point of the ball would instantaneously change by 1cm, and the ball would swing noticeably with a period of about two seconds and a peak speed of well over a centimeter per second.
If instead the plane started with an acceleration of 0.02g, but reduced its acceleration gradually over a period of a minute, the ball's initial displacement would be about twice as big as in the earlier example, but it would take a minute to reach vertical, and its maximum horizontal speed would be an order of magnitude below that of the earlier example.
The human vestibular system is a bit more complicated than a hanging ball on a string, but similar principles apply.

Re "changes in acceleration": It goes by the name jerk... ("5. (physics, engineering) The rate of change in acceleration with respect to time.")

@PeterMortensen  jerk oit is, but it conveys the wrong impression to all but those who deal with it. The next derivative after that is jolt or jounce or, importantly, snap. That's important because it allows the next two derivatives after that to be crackle and pop. Really :-) .  https://wiki2.org/en/Jerk_(physics) . || Sime of Larry Niven's ships had 'jolt meters'. Whether they meant what they should have is moot.

@PeterMortensen  Niven Pournelle use below. They use the term jolt correctly. In the linked passage below a ship increases acceleration from rest to 3g. Jolt alarms warn of the jolt conditions then stop as they settle at a steady 3 g.
A nice account of a space battle using the well integrated assumption set their story is based on.  https://www.baen.com/Chapters/1011250018/The_Mote_in_Gods_Eye.htm

@RussellMcMahon: I'd not learned the term "jerk" in that usage until after I was out of college, and wanted to avoid any unfavorable impressions or associations the term might have to anyone.

Part of the answer is that even violent control surface action is not felt if the result is level flight. While you may have significant changes in the vertical airspeed, corresponding to significantly changing lift forces (for example in order to counteract up- or downdrafts), ideally the airplane experiences little change in vertical ground speed: It stays level in space and is not accelerated much. In fact, maintaining level flight was the very reason for the control surface action.
The same is true for the adjustments due to the weight loss from expelled fuel. Ideally, the reduced lift forces stay equal to the reduced gravitational forces so that the result is zero vertical acceleration and hence level flight (except perhaps minor, slow attitude changes). The cabin is not accelerated so that nothing is felt.


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How does a shockwave form from a subsonic upstream?
How does a shockwave form over say an aerofoil from a subsonic upstream?
Wouldn’t the flow have to be supersonic before it ‘shocks’?
Also, does the flow in a shockwave jump from let’s say it is at M=1.2 (before shockwave), to M<1 after please? Or gradually decrease?

A subsonic velocity doesn't mean air is subsonic everywhere along the airfoil. Lift is created by accelerating air. Related: Do commercial airliners use supercritical airfoils?

Hi, yeah I see what you’re saying, thanks.

The airfoil accelerates the flow such that there is a region of supersonic flow which then can shock down.
The flow through a shock does experience an abrupt jump in properties.  Velocity, Mach, density, pressure, and temperature all change very quickly.
Some other properties do not jump.  For example, mass flow per area normal to the shock does not jump.

Hello! But how does the shockwave actually form in flow that is originally subsonic (but high speed like M=0.9) please? I couldn’t find a nice explanation of shockwaves online

The flow first gradually accelerates to supersonic.  Then it encounters an adverse pressure gradient that wants to cause it to slow down.  However, information can not propagate forward in a supersonic flow, so a shock is triggered.
There is a ton of information on shockwaves available online and in books.  Keep looking.

Hello, thank you for the reply. So the adverse pressure gradient is like the catalyst that makes it jump from say M=1.3 (before the shockwave) to subsonic please? But how does it make the shockwave happen so quick please?

@James shockwaves always happen quickly - otherwise they wouldn't be a shock.    Disturbances in a flow travel at the characteristic wave speed. For this kind of problem, that is the speed of sound. When the flow is faster than the speed of sound, that means that disturbances (information) can not flow upstream. So, when the flow encounters something (downstream) that requires a change (a blockage, a turn, a pressure gradient), the flow must react abruptly. Conversely, in subsonic flow, the disturbance can propagate forward such that it can be 'felt' early and a gradual adjustment can be made.

You’re super clever, thank you rob!!


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Are there FAA regulations for designing new aircraft with pets in cabin?
I understand that these are FAA regulations on pets flying in-cabin on a commercial plane: https://www.faa.gov/travelers/fly_pets/cabin_pets
Let's say I wanted to design a new type of commercial aircraft that is specifically designed to accommodate pets in cabin. For example, built-in kennels for small pets in the cabin, or leash/harness hookups for larger dogs to sit by their owners, etc. Would I start with the new design, or would I follow existing FAA regulations on new aircraft design regarding pets in cabin?
I'm not saying this is feasible or that I personally have the skills required to design a new aircraft, I'm just trying to understand. For context, I own a software company that builds tools for travelers with pets. Thank you

As pointed out in Ralph J's answer, I think that you may have made your question overly ambitious by limiting it to designing a new aircraft. Airliner construction and interior design are two separate functions, and it is common for airline interiors for existing aircraft to be redesigned either by the manufacturer or by airlines or by third party companies.  I think your question would be improved by specifying airliner interior design instead of aircraft design, unless you feel that existing airframe designs will not accommodate the ideas that you have in mind.

You mentioned pets but probably a larger issue that is looming is the greatly increased number of service dogs in use, now that more people are using service dogs for emotional support, as evidenced by an increased number of dogs in grocery stores and restaurants, etc. How many of these are actual service dogs is hard to determine since apparently in many jurisdictions stores are only allowed to ask a dog owner if it's a service dog, they are not allowed to ask what type of service dog or require any proof or documentation.  On airplanes service dogs do not have the same restrictions as pets.

Great point about service animals as well

IATA (operators association) regulates live animals transportation, and IATA Live Animals Regulations (LAR).

Designing a new aircraft is an unimaginably expensive & time-consuming process. Getting approval to install hardware like kennels in an existing aircraft is merely an expensive & time consuming process.
Unless there is something about your concept for kennels & such that (a) cannot be accommodated in existing aircraft, and (b) will be so popular that you can recoup all the design & development cost of a whole new airframe, then modifying an existing design is certainly the better way to go.

Add to the Degree of Difficulty - Humans that are allergic to pet dander.

Thank you Ralph J and Steve Pemberton for the context around airliner interior design; I did not realize that was an option. That is incredibly helpful and yes, I can definitely see how it would make much more sense to modify an existing design. Really appreciate the insight.

WPNSGuy - I noticed in the FAA document that the OP linked to that they pretty much dismissed pet dander objections, saying "You will still be exposed to pet dander on every flight, even without any pets in the passenger cabin. This is because most allergens are carried into the cabin on the clothes of other passengers." Seems pretty dismissive though since I would assume that there is a big difference between sitting next to someone who owns a pet, and sitting next to someone who has a terrier on their lap.

@StevePemberton the fact that airlines don't require passengers to be decontaminated for all known allergens before they're allowed to board a flight, and airports don't do so at the checkin desks, pretty much proves the FAA's point :)


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Engine Performance Data for GE F414-400
I am currently in Capstone for ASE, and for our course we have been tasked with reverse engineering the X-59 QueSST created by NASA and Lockheed Martin. I am the propulsion lead for our team and I have been able to gather the bare minimum specifications of the engine like thrust, SFC, pressure ratios, and weights and such. I am wanting to run a parametric cycle analysis on the mixed exhaust turbofan engines and to do this per Mattingly's PCA method in aircraft engine design book, there are a lot of input parameters such as the bleed air flow rate, mixing ratios, and bunch of other specific variables. Does anyone have engine performance data for the GE F414-400 engine? Any information on it's design specifications or actual data of the installed or uninstalled engine would be a gold mine in verifying that my PCA of the engine is accurate. Currently, I am using general values for all turbofan engines for the variables I can't find. Is there any website or resource that contains this type of data? Thanks in advance for any assistance you may be able to provide.


text:
What department does an airline pilot fall under?
On LinkedIn, I saw on the Page for the Harry Reid International Airport there were charts for 346 members, 157 of them working in Operations, 43 for Administrative, 37 for Business Development, 25 for Information Technology, 25 for Engineering, 21 for Customer Success and Support, 12 for Finance, 11 for Military and Protective Services, 11 for Arts and Design, 10 for Accounting, 10 for Media and Communication, 10 for Sales, 9 for Community and Social Services, 7 for Human Resources, and 6 for Legal. I'm wondering which of these would an airline pilot would be categorized as.

Airline pilots do not work for airports, they work for the airline companies that use airports.  They would not appear, or be counted in airport employee rosters or organizational charts.

Airline pilots work for an airlines not airports, but for argument's sake, if an airport authority had pilots on staff flying aircraft owned by the airport authority for some reason, say in a corporate transport role (not aware of any, but possible I suppose), they would be under Operations in the org chart, and under that you would expect see a sub group called Flight Operations where the pilots are listed.  Same as an airline.


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Air pressure in cold air vs warm air
I’m a little confused here . Studying for ppl exam it is stated that cold air is at higher pressure than warm air rising up, so the flow is from cold to warm. But then again , when flying from warm air to colder air , the pressure in the warmer air will be higher , leading to altimeter error .
My question: is the whole “ colder air with higher density is at higher pressure flowing to replace warmer , less dense air has to do with flow? Is that why there is a discrepancy ?

The surface pressure is indeed lower over a warm surface, but the pressure lapse rate is also reduced. As a result, in the upper troposphere, the situation reverses and the pressure is higher over a warm surface than over a cold one. Here is a related answer by me on this topic: https://earthscience.stackexchange.com/a/25491/29450

If you think of balloons, hot air has the same pressure but lower density.  Lower density creates buoyancy.  Hot air at the same pressure can rise into cooler air.

Thank you very much . I get it now

I have another question : with the rising of air mass and expansion in the equator , why is there no cloud formation / storms , etc there ?

Air flow (wind) is always from higher pressure to lower pressure.
Pressure is dependent on temperature and density.
"Temperature" is essentially how fast air molecules are moving.  Colder air will have a higher pressure gradient with altitude because gravity holds more of the "slower" air molecules closer to earth.
A most extreme example of this would be to cool the air down until it liquefies, leaving a puddle with little else above it.
Warm air will have a lower pressure gradient with altitude because more of the "faster" air molecules can populate the higher end of the vertical air column.
These discrepancies are why we must always convert pressure altitude to density altitude, which is most directly related to aircraft performance.
Because the altimeter works on pressure only, corrections  may be necessary for both local pressure and, if there is a large deviation from standard temperature, local temperature.
This is especially true for very cold temperatures, which can cause the altimeter to read higher than actual altitude.


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Why does Vx increase with altitude and Vy decrease?
As an airplane climbs, the best angle of climb airspeed (Vx) increases and the best rate of climb airspeed (Vy) decreases.
I can't figure it out, why does it happen?
I Hope someone explains it to me
I appreciate

Appears to be answered here https://aviation.stackexchange.com/a/34683/34686

That answer isn't fully baked.  At a minimum, Vx is always slower than Vy.  The figure clearly shows them crossing.

@RobMcDonald -- I don't think the linked answer is in error.  Note that the vertical axis is not altitude.  But yeah I can agree that it doesn't appear to answer the present question.

@quietflyer Good catch.

Vx is all about using higher angle of attack (slower airspeed) and excess thrust to climb.  The more excess thrust one has, the steeper the angle one can climb.
As altitude increases, thrust output decreases.  This is why the plane must lower its angle of climb, reduce its angle of attack, and go faster
in order to continue climbing.
Vy is all about using a lower angle of attack and greater airspeed to climb.  As thrust output decreases, the aircraft must increase its AoA and lower its airspeed to continue climbing.
At the absolute ceiling altitude, only the most efficient combination of (diminished) thrust, airspeed, and angle of attack will allow level flight.  This is where Vx and Vy converge.

The details of this depend on the details of your propulsion model for your aircraft.  I.e. how do maximum throttle thrust/power change with airspeed and altitude.
You also need to pay close attention to equivalent vs true airspeed.
$V_y$ is always greater or equal than $V_x$.  However, at absolute ceiling, they are equal (and the climb angle and rate of climb are both zero).
So, to start apart and end up together, they either have to increase/decrease, both increase (but at different rates), or both decrease (but at different rates).
For a jet aircraft, we usually approximate the thrust available as constant with airspeed.  In this case, Vx will occur at the speed for best L/D, which will be a fixed Ve.  In order for Vy to start off greater, but to eventually equal Vx, Vy must decrease with altitude (in terms of Ve).
For a propeller aircraft, we usually approximate the power available as constant with airspeed.  In this case, Vy will occur at the speed for minimum power required, which is the speed for best $C_L^{3/2}/C_D$.  Which occurs at a fixed lift coefficient and therefore also occurs at a constant $V_e$ with altitude.  So, in order for Vx to start off less than, but to eventually equal Vy, Vx must increase with altitude (in terms of Ve).
In reality, neither of these approximations is perfect, so the truth generally lies somewhere in-between.  However, working it out to great precision requires very detailed models of the propulsion system.